well I am pretty sure the actual sewage system is the same pipes for rain water and household water.

dunno, its what my dad told me. It was the 90ies in Germany..

I am aware of two approaches for a less wasteful approach here:

1. using a rain water cistern for service water
2. using almost clean secondary water for service water (like from the washing machine).

My father contemplated using (1) in our house when he had it built but was discouraged because instead of only having one water meter in the house he would have had to get a second water meter to measure the amount of water that goes into the sewage (normally its assumed to be the same as the fresh water).